Anti-reflection coatings are used in solar cells to increase absorption of incident light and thereby increase the output and efficiency of the solar cell. Flat solar cell designs have been widely used and anti-reflection coating processes for such designs are well known. In prior art processes, anti-reflection coatings are applied by chemical vapor deposition.
Recent solar cell advances have resulted in the use of substantially spherical silicon in a solar cell array, referred to as a spheral solar cell. Each of several silicon spheres is embedded in an aluminum foil such that only a hemispherical surface is exposed to receive incident light while the aluminum foil serves to reflect light to the silicon spheres.
The aluminum foil surface is advantageous for reflecting light to the silicon spheres which are coated with an anti-reflective coating on the silicon spheres. Thus, the light received by the spheres includes light reflected from the aluminum foil. However, anti-reflection coating that is deposited on the aluminum foil as a result of coating of the silicon spheres reduces the light reflected to the silicon spheres. This results in a decrease in incident light on the silicon spheres resulting in decreased output and efficiency of the solar cell.
One particular spheral solar cell array and a method of fabricating the solar cell array are disclosed in U.S. patent application Ser. No. 09/988,988 (publication no. US 2002/0096206 A1), published Jul. 25, 2002. This application teaches a plurality of substantially spherical photoelectric conversion elements, each element being mounted in each of a plurality of recesses of a support for reflecting light to the elements. The elements include an n-type amorphous silicon covered with a p-type amorphous SiC layer, forming a p-n junction. The recesses are hexagonally shaped in a hexagonal array and the elements are mounted in respective openings in the hexagonal recesses such that the p-type amorphous SiC layer contacts the support and part of the elements extend through the support. Mechanical grinding of the parts of the elements that extend through the support exposes the n-type amorphous silicon. This particular solar cell array suffers from disadvantages, however, in that there is no anti-reflection coating to reduce the light reflected from the elements.
U.S. Pat. No. 6,355,873 discloses a spheral solar cell array of a plurality of substantially spherical photoelectric conversion elements including an p-type material covered by an n-type material. The elements are pressed into a wire mesh such that the wire mesh contacts the n-type material and each element is polished to remove a portion thereof and expose the p-type material. The elements, embedded in the wire mesh, are mounted in concave dimples of a dimpled sheet of aluminum foil such that a leg in each dimple contacts the p-type material. Again, this solar cell array does not include an anti-reflection coating to reduce the light reflected from the elements. Furthermore, this solar cell array must be manufactured such that contact between each leg and the n-type material of each of the elements is avoided.
One prior art method of applying an anti-reflection coating to a spheral solar cell is disclosed in U.S. Pat. No. 5,081,069, issued Jan. 14, 1992 to Parker et al. The anti-reflection coating is applied to the silicon spheres in a chemical vapor deposition (CVD) reactor chamber by introducing heated titanium isopropoxide into the CVD reactor chamber at atmospheric pressure, through reactor nozzles. This method relies on controlled decomposition of metal alkoxide with the aim of providing a uniform coating of metal oxide (titanium dioxide). This method suffers from many disadvantages, however. For example, metal oxide is not substantially uniformly deposited on the surface of the silicon spheres, resulting in a relatively thick layer of anti-reflection coating at the top of the silicon spheres and a relatively thin layer of anti-reflection coating on the underside of the silicon spheres. Also, densification of the coating to achieve desirable refractive indices is limited to temperatures below about 577 degrees Celsius. Higher temperatures lead to undesirable formation of aluminum silicide liquid phases at the aluminum-silicon bond, thereby degrading the finished cell.
It is therefore desirable to provide a method of fabricating a photovoltaic solar cell, that obviates or mitigates at least some of the disadvantages of the prior art.